The use of programmable variable resistance materials in electronic devices is known in the art. The chalcogenides are an important class of programmable variable resistance materials. The principles of operation of chalcogenide (and phase change) materials and devices are described in U.S. Pat. No. 5,296,716, U.S. Pat. No. 5,341,328, U.S. Pat. No. 5,359,205, and U.S. Pat. No. 7,227,170, all to Ovshinsky et al., which are incorporated herein by reference. These patents are believed to evidence the state of the prior art and to represent current theory of operation and function of phase change materials and chalcogenide-based memories known to those skilled in the art.
Briefly, variable resistance materials are materials that can be caused to change physical or electronic state, and therefore resistivity level, in response to an electrical input stimulus. By way of example, phase-change materials (many chalcogenide) may be electrically stimulated to transform among structural states ranging from a predominantly crystalline state to a predominantly amorphous state. By controlling the amount of electrical energy applied to a chalcogenide phase-change material, the relative proportions of crystalline and amorphous phase content can be continuously varied from a low crystalline phase volume fraction to a high crystalline phase volume fraction. The resistivity of a chalcogenide phase-change material correlates with the crystalline phase volume fraction and progressively decreases as the crystalline phase volume fraction increases. A chalcogenide phase-change material may be predictably placed in a particular resistivity state by running a current of a certain amperage for a certain duration through it. The resistivity state so fixed will remain unchanged unless and until a current having a different amperage or duration within the programming range is run through the material.
Because of these unique characteristics, variable resistance memory materials may be used in memory cells for storing data in binary or higher-based digital systems. Such memory cells will normally include a memory element that is capable of assuming multiple, generally stable, states in response to the application of a stimulus. In most cases, the stimulus will be a voltage differential applied across the element so as to cause a predetermined current to flow through the memory element. A chalcogenide-based memory cell will typically include a chalcogenide memory element utilizing a chalcogenide phase-change material for storing data and an access element, coupled to the memory element, for use in programming and sensing the stored data. The access element may be, in one embodiment, a diode.
To achieve high density storage of data, memory arrays comprising a multitude of chalcogenide memory elements may be fabricated. In a memory array, a grid of conductive row lines (wordlines) and column lines (digit lines or bit lines) is formed in which a series combination of an access element and a chalcogenide memory cell is located at each junction of a row line and column line. The row lines and columns lines are connected to external circuitry (such as drivers or sense amplifiers) and individual memory cells are programmed or read by selective application of voltages to the row line and column line between which the memory cell is interconnected. Selection of the row line and column line of a particular memory cell produces a voltage differential that activates the access element, thus enabling current to pass through the memory element. Access elements at non-selected junctions of the array prevent stray current from altering the state of memory elements located at non-selected junctions.
Because of the unique operating characteristics of memories based on variable resistance memory elements, control of current flow is crucial to facilitate programming. Programming of chalcogenide phase-change materials, for example, requires high current densities. In this regard, it is desirable that a chalcogenide-based memory cell include a diode or other access element capable of permitting a large current flow in the forward direction to program the memory cell. Conventional junction diode structures that are capable of supplying the necessary current require a significant number of complex processing steps to create compared to a deposited thin film diode. Accordingly, there is a need for a stable and easily manufactured access device that can meet the performance requirements of chalcogenide-based memory cells, while permitting fabrication of small footprint devices with minimal added processing steps to achieve a cost effective high density memory arrays.